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Secondary injection testing proves that a protection relay measures, decides and trips correctly when it is fed the currents and voltages its instrument transformers would deliver. The set injects those quantities at the relay terminals, so the relay input, the panel wiring from the injection point, and the trip logic are exercised as one chain.
The work belongs after panel wiring is complete and before the plant is energized, when the instrument transformer circuits can be isolated safely and a fault simulated with no risk to the network.
What secondary injection testing proves
A relay only sees the secondary side of its instrument transformers. When a set injects a known quantity, the value the relay reports should match it, scaled by the ratio and vector group programmed into the device. That comparison separates two different problems: a relay that is set incorrectly, and a circuit that delivers the wrong signal to a relay that is configured correctly.
Injection also proves the binary side. The set supplies the auxiliary DC and drives the binary inputs, then watches the output contacts, so the logic that combines elements into a trip decision is verified and not only each element alone.
How a secondary injection test is staged
The sequence is the same for electromechanical, static and numerical relays. Leaving a step out is why a result cannot be repeated.
- Isolate. Open the CT and VT secondary test links and short the CT secondary before touching the circuit. A CT secondary must never be left open while primary current can flow.
- Record the starting state. Note the relay model, firmware version and the settings read back from the relay itself. A result is only meaningful against a known settings set.
- Connect at the relay terminals. Wire the current channels in series with the relay current inputs and the voltage channels across the voltage inputs, and verify polarity before applying current.
- Run the test plan. Step through pickup, timing, characteristic and logic tests, recording the applied quantity and the relay response at each point.
- Restore. Remove the leads, remake the test links and prove continuity to the instrument transformer. A link left open disables protection.
What secondary injection testing verifies
| Test | Injected quantity | What a pass proves |
|---|---|---|
| Pickup | A slowly rising current or voltage | The measured threshold matches the setting |
| Timing | A step to a value well above pickup | Operating time follows the declared curve |
| Directional | Current at a defined phase angle | The element operates forward only and blocks reverse faults |
| Differential | Currents on every winding or end | The bias characteristic holds and stays stable through a fault |
| Logic | Binary inputs and the auxiliary supply | The trip matrix and signaling match the scheme drawing |
A single pass proves little on its own. The useful evidence is the shape of the result across the range tested, plus a repeat at a point close to the boundary.
Common mistakes and how they show up
- Mixing secondary and primary quantities. A setting may be expressed on the primary side while the set works on the secondary side. Convert with the CT ratio so every point stays on one basis.
- Ignoring polarity. Reversed current channels make a directional or differential element behave exactly like a miswired CT. Confirm the convention and the star-point reference before the first test.
- Using the wrong reference. A phase-to-phase voltage is not the phase-to-neutral quantity a setting assumes. State the reference next to every recorded value.
- Testing against the settings sheet instead of the relay. If the paper and the relay disagree, resolve that first; results taken against an unknown settings set will not be accepted.
Frequently asked questions
How is secondary injection different from primary injection?
Primary injection drives current through the complete primary circuit, including the CT ratio and its saturation behavior. Secondary injection works at the relay terminals and proves the relay and its wiring, but not the CT itself. The two are complementary.
Which tests must be done at the relay terminals?
Any test whose purpose is to prove the relay or its panel wiring belongs at the terminals: pickup and timing, characteristic and logic checks, and the trip matrix. Tests that characterize the instrument transformer belong at the transformer.
How do you prove a trip circuit end to end?
Injection proves the relay closes its output contact, not that the trip coil energizes. Energize the trip path from the auxiliary supply with the breaker isolated, confirm the coil current and the breaker trip indication, and record the result.
Where VA-TEK test sets fit
Most secondary injection testing uses a three-phase set. The VAE-430 provides 3×0–40 A AC and 4×0–120 V AC with 8 binary inputs and 4 binary outputs, and up to 1×0–120 A in parallel for high-burden relays. Schemes needing six currents at once, such as a transformer differential tested from both ends, call for the VAE-660 with 6×0–30 A AC channels and 1×0–180 A single-phase output at 1000 VA. Both appear on the secondary test instruments family page.
Two companion articles go further: how to choose a relay test set covers channel count and power, and when to use six-phase relay testing explains the schemes that need the extra channels. To plan a commissioning program, contact VA-TEK or send the scheme details through the quote request form.
